1. Field of the Invention
Embodiments of the invention relate to methods and compositions for formation of zeolite impregnated with titanium dioxide (TiO2). Such modified zeolites may be useful, for example, as a substitute or extender for titanium dioxide used in coatings.
2. Description of the Related Art
Various uses and formulations of titanium dioxide and substitutes for titanium dioxide are reported in the art. For example, the DuPont™ Company provides a variety of titanium dioxide coatings for various applications under the trade name Ti-PURE, as reported, for example, in the DUPONT Ti-PURE Titanium Dioxide for Coatings Booklet, which is incorporated by reference herein. (Both DUPONT and Ti-PURE are trademarks of DuPont.) Use of kaolin extenders for these and other titanium dioxide products are reported in the paper “New Generation Kaolin-Based Pigment Extenders,” by L. Ashek, published in 2003 in Surface Coatings International. Titanium dioxide is used in significant amounts in the decorative coatings sector, where it is used for the protection and decoration of buildings. In fact, titanium dioxide is so important that it is sometimes called “white gold,” because it is the raw material that contributes most to the increase in opacity and coverage of water-based paints. Titanium dioxide has a high refractive index (2.73 for rutile, 2.55 for anatase). This causes it to act as a white pigment, scattering or bending light. With enough titanium dioxide in a paint or coating, almost all visible light is reflected causing the coating to appear white, bright, and opaque.
Titanium dioxide is typically provided in a delivery vehicle. This may be, for example, water, vinyl resin, or acrylic resin. These compounds may have refractive indexes between 1.3 and 1.49. The greater the difference between the refractive indexes of the delivery vehicle and the refractive index of the titanium dioxide, the greater the light-scattering effect.
Size of titanium dioxide may be relevant to its efficacy. For example, typical titanium dioxide particles have a particle diameter of between 0.2 and 0.4 microns. This is about half the wavelength of light at which brightness and opacity are measured.
Dispersion of titanium dioxide is also important to performance as a brightener and opacity-creating agent. Flocculation of titanium dioxide is undesirable because it results in a larger effective particle size, decreasing the refractive effect.
Because of the expense associated with the use of titanium dioxide, effort has been expended to find substitutes or extenders. The choice of extender depends largely on the properties to be enhanced or controlled in a paint formulation. Blends of extenders may be used depending on the properties that are desired. The primary classes of titanium dioxide extenders are carbonates, silicates, sulfates, kaolins and oxides. Their particle sizes typically range from 0.01 to 44 microns. A high-gloss white paint usually contains only TiO2; a semi gloss paint contains TiO2 and some extender pigments; a flat paint contains TiO2 but has a high extender content.
The ability of a material to act as an extender to titanium dioxide is governed in part by the size of the particles in the extender. Extenders that are too large may crowd the titanium dioxide. Extenders that are too small may overly disperse the titanium dioxide. In either case the efficacy of the overall composition may be diminished.
Coatings properties are directly related to pigment volume concentration, or PVC. These properties include, for example, gloss, permeability, porosity, hiding power, tinting strength and undertone. Because a dry paint film is a three-dimensional structure, the volume relationships among its components have an impact on paint performance. PVC is the ratio, by volume, of all pigments in the paint to total non-volatiles in the paint.
At a particular PVC, called the critical pigment volume concentration (CPVC), many physical and optical properties of paint change abruptly. Typically CPVC is the PVC where there is just sufficient binder to coat pigment surfaces and provide a continuous phase throughout the film.% PVC={Pigment volume(TiO2+extenders)}/{Pigment volume+volume of binders}*100
Typically, PVC calculation has included only the pigment and the binder. It not account for the air voids present in the coating. In many paint coatings, air voids are intentionally added using a structured pigment. This improves optical properties of the pigment. To account for volume of air voids, the PVC equation would be altered to include the void volume.
As air voids are incorporated into a paint film as a result of formulating highly pigmented coatings above the CPVC, the average refractive index of the vehicle matrix decreases. This increases the refractive index difference between the pigment and surrounding medium, increasing light scattering. Formulators often use dry flat hiding to improve hiding of low-gloss flat interior architectural finishes.
In addition to TiO2 and a delivery vehicle, many paints contain extender pigments. White extender pigments are mineral compounds of relatively low refractive index. They differ in composition, size and shape. White extender pigments develop very little hiding in gloss and semi-gloss paints. However, they contribute dry-flat hiding (air-pigment interface) to paints at low cost and are used to control gloss, texture, suspension, and viscosity.
According to the 2008 Chemicals Economics Handbook for Titanium Dioxide, In the United States, 50-60% of TiO2 is consumed in paints and coatings, 20-25% in plastics, 10-15% in paper and 5-10% in all other uses.
Titanium dioxide is by far the most widely used white pigment in plastics. Virtually all plastics have some applications that require TiO2. The major portion of titanium dioxide pigment consumption is accounted for by the huge commodity thermoplastics, including polyolefins (primarily low-density polyethylenes), polystyrene, polyvinyl chloride (PVC) and ABS (acrylonitrile-butadiene-styrene copolymer). Titanium dioxide pigments are incorporated into plastics to provide whiteness and opacity. In some applications, such as outdoor PVC siding, higher loadings (12%-22%) are used for ultraviolet light protection and improved dimensional properties. White plastic packaging films and containers for food, beverage, personal care and other consumer goods also contain TiO2 to provide an opaque, white, bright appearance to display printed graphics.
The most commonly used pigment for papermaking is kaolin clay, sometimes in combination with calcium carbonate. Titanium dioxide is used in higher-quality products to impart opacity and brightness. Three segments of the paper market account for the bulk of TiO2 consumption: (1) Coated printing and writing papers (accounts for 50% of all TiO2 use), which includes high-end magazines and advertisements. TiO2 allows magazines to take advantage of lighter weight papers while maintaining a quality product with clean, white, bright pages, and allows catalogers to display their products on pages that maximize the true look of their merchandise without the degrading interference of print show-through. (2) Bleached and coated paperboard (25%), including packaging used for foods and beverages, detergents and tissues. Coated board, coated recycled board, and whitetop corrugated linerboard packaging all use TiO2 in their surface coating to provide the highly opaque, bright, clean, white coated surface enabling high quality printed graphics. (3) Uncoated printing and writing papers (20%) which includes copy paper and form bonds.
Titanium dioxide pigments are also used in a number of natural and synthetic elastomer products. The two largest elastomer markets for TiO2 are white sidewalls for passenger tires and rubber footwear. Other elastomer products that contain TiO2 include floor mats, gloves, rainwear, wall coverings and sports equipment. The TiO2 content of white sidewall formulations varies from 8% to almost 20% for applications in which high ozone resistance is desired. Up to 15% TiO2 is recommended for white stock thermoplastic elastomers, for UV protection as well as improved color. Primarily anatase-grade TiO2 is used by the elastomer industry. Consumption of TiO2 for tires is believed to account for about one-half of elastomeric consumption of TiO2.
Some white printing inks may contain as much as 55% TiO2 by weight (primarily rutile), which is substantially more than typical white paints. Many light-colored inks also require a substantial amount of TiO2 for their formulation. All of the major ink types—flexographic, rotogravure, letterpress and lithographic—consume some TiO2, although lithographic and specialty ink categories, such as screen process inks, are minor markets for TiO2.
Titanium dioxide is used as a component of porcelain enamels and glazes for metals, ceramics and specialty glasses. In general, non pigmentary grades of TiO2 are used in ceramic applications; they differ from pigmentary grades in that they have larger particle sizes and the surface of the particle is untreated. Titanium dioxide is incorporated into frits (the glassy material that is the major component of porcelain enamels and glazes) in varying amounts up to 20%. Frits are applied to metal substrates and then fired at around 700° C.; upon recrystallization, the coating appears white and opaque. TiO2 is also incorporated into white dry-process enamels for cast iron. The TiO2 content of these enamels is only 4-8%. If pastels or colored enamels are desired, rutile mineral is used to supply the TiO2 because it is less expensive. TiO2 in glazes for ceramics can serve as an opacifier, to provide resistance to acids and for special surface texture (semimattes). TiO2 pigment is incorporated into glasses, including reflective beads, to increase opacity. Other TiO2 markets are as follows:
Floor coverings. This category includes titanium dioxide consumed only in resilient floor tiles and sheet goods. Typically, these floor coverings contain 2-4% TiO2 by weight, although some products have a much lower content.
Coated fabrics and textiles. Rutile-grade pigments are generally preferred for the coated fabric market, which includes artificial leather, oil cloth, upholstery material and wall coverings.
Roofing granules. TiO2 is also used as a component of roofing materials to increase light reflectance and to provide flame retardance and thermal insulation. Among the many other miscellaneous applications of titanium dioxide pigments are adhesives and sealants, artist's colors and crayons, building materials (e.g., ceiling tiles), cement-curing aids, natural leather, shoe dressings and soap.
Specialty cements or concretes. Where whiteness in final appearance is required, these compositions may benefit from inclusion of TiO2.
Some grades of TiO2 comply with FDA requirements and are used as food color additives and by the cosmetics industry, in which TiO2 may comprise as much as 20% by weight of some products such as face makeup; lower percentages are used for sunscreens, eye shadow, lip makeup and other products. There has been some increase in use in the sunscreen industry as TiO2 is more effective at providing protection from UV-A radiation.
Unfortunately, use of titanium dioxide imposes a high cost for raw materials; in some cases it may be ten times more expensive than other raw materials in the same formulation. Therefore, having more economical technical options is of great interest to this market. Currently, it is very common to use other raw materials to substitute titanium dioxide, even if only partially. These solutions may be of limited utility, since the coverage that they offer may not be of quality (either related to opacity or coverage or both) of titanium dioxide.
Use of titanium dioxide in combination with zeolites has been reported. U.S. Pat. No. 4,220,567 to Kindervater et al. for “Stabilizing Pigmented Lacquers with Zeolites” reports the use of zeolites in lacquer containing iron oxide or titanium dioxide pigments to improve the dispersion and prevent flocculation of said pigments or other fillers.
U.S. Pat. No. 4,433,196 to Yang et al. for “Color Precursor Removal from Detergent Range Alkyl Benzenes” reports a method for removing color precursors from detergent range alkyl benzenes with an adsorbent. The adsorbent contains a bauxite clay with titanium dioxide, mixing said clay with sulfuric acid followed by calcination, and a crystalline zeolite.
U.S. Pat. No. 4,752,341 to S. Rock for “Pigment System for Paper” reports a combination of titanium dioxide and zeolite providing improved optical properties when included in paper. Zeolite A is modified to exchange sodium for calcium and blended with titanium dioxide pigment.
U.S. Pat. No. 5,385,753 to Hu et al. for “Process for Reactively Coating Particles” reports a process to make TiO2-Zeolite coated particles to be used as a replacement for TiO2 pigment. The product is obtained from a natural or synthetic zeolite reaction with titanium tetra chloride (or titanium ether or esters) in an inert atmosphere such as nitrogen, and in a water-immiscible liquid environment such as heptanes.
U.S. Pat. No. 5,474,754 to Saxton et al. for “Preparation of an Aluminosilicotitanate Isomorphous with Zeolite Beta” reports a method for synthesizing molecular sieves isomorphous with zeolite beta with titanium atoms on their framework. The product is obtained by reacting silicon/aluminum/titanium compounds in acid condition followed by a treatment with quaternary tetraethyl ammonium species in a solvent to form an impregnated co-gel.
U.S. Pat. No. 6,569,394 to Fischer et al. for “Catalyst Body and Process for Breaking Down Nitrogen Oxides” reports a catalyst body for breaking down nitrogen oxides with an active material that contains zeolite and titanium dioxide. The zeolite is a hydrogen-ion-exchanged in which the cations have been exchanged by hydrogen ions. This zeolite is mixed with an active component containing titanium dioxide.
Zeolites are micro porous crystalline solids with well-defined structures. Generally they contain silicon, aluminum and oxygen in their framework and cations (such as Na+, K+, Ca2+, Mg2+ and others), water and/or other molecules within their pores. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Many zeolites naturally as minerals and are extensively mined in many parts of the world. Others are synthetic, and are made commercially for specific uses. An example mineral formula for a zeolite is: Na2Al2Si3O10-2H2O, which is the formula for natrolite.
There are several types of synthetic zeolites that form by a process of slow crystallization of a silica-alumina gel in the presence of alkalis and organic templates. The product properties depend on reaction mixture composition, pH of the system, operating temperature, pre-reaction ‘seeding’ time, reaction time as well as the templates used. Preparation of synthetic zeolites suitable for use in embodiments of the invention is shown, for example, in U.S. Pat. No. 4,661,334, to Latounnette, et al. (“Preparation of Zeolites 4A and/or 13X”); U.S. Pat. No. 4,649,036 to Pastorello, et al. (“Process for the Manufacture of Zeolites 4A . . . ”); U.S. Pat. No. 5,487,882 to Hu, et al. (“Process for Preparation of Zeolite 13X”); U.S. Pat. No. 6,258,768, to Araya (“Zeolite P . . . ”); and U.S. Pat. No. 4,264,562, to Kostinko (“Method of Producing Zeolite Y”).
Synthetic zeolites hold some key advantages over their natural analogs. The synthetics can, of course, be manufactured in a uniform, phase-pure state. It is also possible to manufacture desirable zeolite structures which do not appear in nature. Zeolite A is a well-known example. Examples of synthetic zeolites are the A, P, X and/or Y types. One example of a type A zeolite has the chemical formula Na2O:2SiO2:Al2O3:3.94H2O, wherein the quantity of Na2O is 17%, Al2O3 is 29%, SiO2 is 34% and H2O is 20%. U.S. Pat. No. 4,264,562, to Kostinko gives a description of different synthetic zeolite types.
Zeolites can also be made from kaolins or other sources containing alumina. Kaolin clays or pigments are naturally occurring hydrated aluminum silicates of the approximate formula Al2O3.2SiO2.XH2O, wherein X is usually 2. Kaolinite, nacrite, dickite and halloysite are species of minerals in the kaolin group. U.S. Pat. No. 6,656,347, to Stockwell et al. “Structurally Enhanced Cracking Catalysts” describes the preparation of zeolite from ultra fine calcined hydrous kaolin, with or without the presence of sodium silicate and sodium (or potassium) hydroxides to form zeolite.
The general formula for zeolites can be expressed by Na2O:χSiO2:Al2O3:γH2O. Zeolite X will have χ=2.5±0.5, Zeolite A will have χ=1.85±0.5, Zeolite Y will have χ=4.5±1.5. U.S. Pat. No. 6,258,768 (Arraya) describes the typical formula for Zeolite P where χ will vary from 1.80 up to 2.66. The water content on the structure, represented by γ is variable and can reach up to 9. Typical values of γ for Zeolite X are 6.2 and Zeolite A is 3.91. In some embodiments the value of γ is in a range of 3 to 9 for synthetic grades.
It would be helpful to have a zeolite that includes titanium dioxide as an impregnated component. By “impregnated” it is meant that the titanium dioxide is incorporated into the crystal structure either as part of the crystal structure or chemically attached to the surface of the crystal, resulting in a strong bond link that would not represent a simple physical mixture of zeolite and titanium dioxide, for example. Such a zeolite could act as an extender or replacement for titanium dioxide in coatings.